Sheila G. Bailey

Effect of strain compensation on quantum dot enhanced GaAs solar cells

GaP tensile strain compensation (SC) layers were introduced into GaAs solar cells enhanced with a five layer stack of InAs quantum dots (QDs). One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current. The strain compensated QD solar cell shows a slight increase in short circuit current compared to a baseline GaAs cell due to sub-GaAs bandgap absorption by the InAs QD. Quantum efficiency and electroluminescence were also measured and provide further insight to the improvements due to SC.

Electrodeposited CdS on CIS pn junctions

We have been investigating the electrochemical deposition of thin films and junctions of cadmium sulfide (CdS) and copper indium diselenide (CIS). We show that it is possible to fabricate pn junctions based on n-type CdS and p-type CIS entirely by electrodeposition. CIS is considered to be one of the best absorber materials for use in polycrystalline thin-film photovoltaic solar cells. CdS provides a closely lattice-matched window layer for CIS. Electrodeposition is a simple and inexpensive method for producing thin-film CdS and CIS. We have produced both p- and n-type CIS thin films, as well as a CdS on CIS pn junction via electrodeposition. Elemental analysis of the CdS and CIS thin films was performed using X-ray photoelectron spectroscopy and energy dispersive spectroscopy. Optical band gaps were determined for these films using optical transmission spectroscopy. Carrier densities of the CIS films as a function of their deposition voltage were determined from capacitance vs. voltage measurements using Al Schottky barriers. Current vs. voltage characteristics were measured for the Al on CIS Schottky barriers and for the CdS on CIS pn junction.

Short circuit current enhancement of GaAs solar cells using strain compensated InAs quantum dots

Tensile strain compensation (SC) layers were introduced into GaAs p-i-n solar cells grown with a five-stack of InAs quantum dots (QDs) within the i-region. The effects of strain within stacked layers of InAs quantum dots (QDs) were investigated using high resolution x-ray diffraction (HRXRD). Analysis of the HRXRD data shows that the average lattice strain is minimized for the optimal SC thickness. One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current when compared to uncompensated devices. The strain compensated 5-layer QD solar cell shows a 0.9 mA/cm2 increase in short circuit current compared to a baseline GaAs cell. Quantum efficiency measurements show this additional current results from photo-generated carriers within the quantum confined material.

Multi-Junction Solar Cell Spectral Tuning with Quantum Dots

We have theoretically analyzed the potential efficiency improvement to multi-junction solar cell efficiencies which are available through the incorporation of quantum dot using detailed balance calculations. We have also experimentally investigated the Stranski-Krastanov growth of self-organized InAs quantum dots and quantum dot arrays on lattice-matched GaAs by metallorganic vapor phase epitaxy (MOVPE). The morphology of the quantum dots were investigated as a function of their growth parameters by atomic force microscopy (AFM). Photoluminescence and optical absorption measurements have demonstrated that the incorporation of InAs quantum dots (QD) into a GaAs structure can provide sub-GaAs bandgap electronic states

Photovoltaic power for a lunar base

A lunar base is an attractive option for space exploration plans early in the next century. The primary options for a lunar base power system are solar and nuclear. This paper details the requirements for a photovoltaic powered lunar base. Topics covered are (1) requirements for power during the lunar day and during the night, (2) solar cells, present and future availability, efficiency, specific power, and temperature sensitivity, (3) storage options for the lunar night, (4) arrays and system integration, and (5) the potential for production of photovoltaic arrays and storage capability from locally available materials.

Growth and Characterization of InAs Quantum Dot Enhanced Photovoltaic Devices

The growth of InAs quantum dots (QDs) by organometallic vapor phase epitaxy (OMVPE) for use in GaAs based photovoltaics devices was investigated. Growth of InAs quantum dots was optimized according to their morphology and photoluminescence using growth temperature and V/III ratio. The optimized InAs QDs had sizes near 7×40 nm with a dot density of 5(±0.5)×10 10 cm -2 . These optimized QDs were incorporated into GaAs based p-i-n solar cell structures. Cells with single and multiple (5x) layers of QDs were embedded in the i-region of the GaAs p-i-n cell structure. An array of 1 cm 2 solar cells was fabricated on these wafers, IV curves collected under 1 sun AM0 conditions, and the spectral response measured from 300-1100 nm. The quantum efficiency for each QD cell clearly shows sub-bandgap conversion, indicating a contribution due to the QDs. Unfortunately, the overarching result of the addition of quantum dots to the baseline p-i-n GaAs cells was a decrease in efficiency. However, the addition of thin GaP strain compensating layers between the QD layers, was found to reduce this efficiency degradation and significantly enhance the subgap conversion in comparison to the un-compensated quantum dot cells.

RECENT ADVANCES IN SOLAR CELL TECHNOLOGY

The advances in solar cell efficiency, radiation tolerance, and cost over the last decade are reviewed. Potential performance of thin-film solar cells in space are discussed, and the cost and the historical trends in production capability of the photovoltaics industry are considered with respect to the requirements of space power systems. Concentrator cells with conversion efficiency over 30%, and nonconcentrating solar cells with efficiency over 25 % are now available, and advanced radiation-tolerant cells and lightweight, thin-film arrays are both being developed. Nonsolar applications of solar cells, including thermophotovoltaics, alpha- and betavoltaics, and laser power receivers, are also discussed.

Minority carrier diffusion length and edge surface-recombination velocity in InP

A scanning electron microscope was used to obtain the electron‐beam‐induced current (EBIC) profiles in InP specimens containing a Schottky barrier perpendicular to the scanned (edge) surface. An independent technique was used to measure the edge surface‐recombination velocity (Vs). These values were used in a fit of the experimental EBIC data with a theoretical expression for normalized EBIC [C. Donolato, Solid State Electron. 25, 1077 (1982)] to obtain the electron (minority carrier) diffusion length (Ln).

Causes of Power-Related Satellite Failures

Satellite on-orbit failures are rare, but costly. Failures cumulatively account for losses that total many billions of dollars. In order to understand the causes of power- system related failures, publicly-available data sources were analyzed to determine the origin of power-system related failures of spacecraft. Data from reported failures and anomalies of commercial and scientific satellites from the period 1990-2006 were analyzed. Military, GPS, and reconnaissance satellites were not included in the data set. The major failures were divided into nine primary categories: Impact or collision induced failures, battery failures, solar array mechanical failures, attitude control failures, failures due to plasma-discharge events, cell failures, other array failures, darkening of glass or solar reflectors, and cell interconnect failure. These failures account for a reported cumulative loss of 4.4 billion dollars. These data were analyzed to show the cause of each type of failure as a percentage of the total number of failures, and the net cost of each type of failure

Electrochemical synthesis of CuInSe2 for thin film devices

Abstract We have been investigating the materials properties and electrical characteristics of Schottky barriers on electrochemically deposited thin film copper indium diselenide (CIS). Electrochemical deposition is a simple and inexpensive technique of producing thin-film CIS. Stoichiometric control of the as-deposited films is provided by the deposition potential. Native defects, arising from stoichiometric deviations, have allowed the production of both p- and n-type CIS thin films from a single aqueous solution. We have compared the crystallinity, composition and electrical characteristics of as-deposited films to similar films annealed in an Argon atmosphere. The crystallinity showed a significant improvement with annealing. Current versus voltage measurements were used to verify the rectifying behavior of the Schottky barriers and determine their barrier heights. A decrease in the barrier height with increasing carrier density was observed. The capacitance versus voltage dependence of Al Schottky contacts was used to determine the carrier densities in the films. Carrier densities were found to increase with deviation from stoichiometry and decrease with annealing temperature.